An electric discharge machine uses an electrode to machine a workpiece by spark erosion. The size and design of electrode used depends on the particular task at hand. It is therefore important to be able to remove and replace electrodes.
It is known to attach an electrode to an electrode coupling. The electrode coupling is designed so that it may be repeatedly attached to and removed from a chuck mounted on an electric discharge machine. The electrode coupling is thus securely yet releasably attachable to the chuck. Chucks and electrode couplings are generally constructed of a strong elastic material such as steel.
FIG. 1 illustrates a known design of chuck 5. The chuck 5 includes a surface 10. For illustrative purposes, x-, y- and z-axes are indicated on FIG. 1. The x-y plane is defined by the surface 10, and the origin of the x-y-z axes is centrally located on the surface 10.
It is important that an electrode be positioned accurately relative to a chuck in both the x- and y-directions. Accordingly, the illustrated chuck 5 includes four spaced apart eccentric protrusions 15 designed to engage corresponding orifices in an electrode coupling. Other known designs of chuck may include more or fewer protrusions, or may include protrusions of different sizes and shapes. The protrusions 15 should be accurately machined in order to ensure that an electrode coupling may be positioned precisely relative to the chuck 5 in the x- and y-directions.
FIGS. 2 and 3 illustrate a first prior art design of electrode coupling 20, and FIGS. 4, 5 and 6 illustrate a second prior art design of electrode coupling 20. With respect to each prior art design, the electrode coupling 20 includes an electrode end 25 to which an electrode may be attached, and a chuck end 30 designed to engage the chuck 6. The coupling 20 includes a centrally located hole 35, and four channels 40 arranged in a substantially X-shaped pattern. The channels 40 are, in fact, arranged in a cross-shaped pattern. For the purposes of the present application, a cross-shaped pattern is considered to be a special case of a substantially X-shaped pattern. An X-shaped pattern includes a cross-shaped pattern, which is a pair of collinear channels perpendicular to another pair of collinear channels. The channels 40 are designed to receive the protrusions 15 such that each protrusion 15 is securely yet releasably held by its corresponding channel 40. Thus, to replace one electrode with another, the electrode coupling 20 is removed from the chuck 5, thereby removing the protrusions 15 from the channels 40. The electrode coupling 20 is then replaced by another electrode coupling 20 to which is attached a different electrode or, alternatively, a different electrode is attached to the same electrode coupling 20 which is then reattached to the chuck 5.
The channels 40 illustrated in FIGS. 2 and 3 are rectangular in cross section, each including two side walls 45 and a floor 47. The distance between the side walls 45 of each channel 40 is such that there is an interference fit between the protrusions 15 and the channels 40. As the electrode coupling 20 is constructed of steel, and as there is an interference fit between the protrusions 15 and the channels 40, the side walls 45 deform elastically at least in the x- and y-directions upon the insertion of the protrusions 15. The side walls are therefore elastically deformable. The forceful interaction between the protrusions 15 and the side walls 45 causes the protrusions 15 to be held securely yet releasably within the channels 40. The electrode coupling 20 is thus securely yet releasably attachable to the chuck 5. However, the contact between the side walls 45 and the protrusions 15 leads to wear of the side walls 45 and the protrusions 15. Such wear, in turn, leads to less accurate positioning of the electrode relative to the chuck 5 in the x- and y-directions, and to less secure attachment of the electrode coupling 20 to the chuck 5.
FIGS. 4, 5 and 6 illustrate a further prior art design of channel 40, which is disclosed in U.S. Pat. No. 4,855,558 to Ramsbro. Referring to FIGS. 4 and 5 in particular, each illustrated channel 40 is rectangular in cross section and includes two side walls 45 having leading edges 50, and a floor 47. Into each side wall 45 extends a recess 55 which runs along the length of the side wall 45. The recesses 55, which curve slightly downward, extend substantially horizontally into the body of the electrode coupling 20. Each recess 55 creates a substantially horizontally oriented lip 60. As the illustrated electrode coupling 20 is constructed of steel, the lips 60 may be elastically bent downward, i.e. essentially in the z-direction, as indicated by the broken line in FIG. 6.
The distance between the side walls 45 is such that there is an interference fit between the protrusions 15 and the channels 40. FIG. 6 illustrates a protrusion 15 inserted into the channel 40 of FIGS. 4 and 5. As there is an interference fit between the protrusion 15 and the channel 40, the protrusion 15 contacts and exerts force on the leading edges 50 of the side wall 40, thus bending the lips 60 downward, and possibly deforming the side walls 45 elastically in the x- and y-directions. The side walls 45 are therefore elastically deformable and downwardly elastically moveable. The forceful interaction between the protrusions 15 and the side walls 45, and particularly the contact between the edges 50 and the protrusions 15, causes the protrusions 15 to be held securely yet releasably within the channels 40. The electrode coupling 20 is thus securely yet releasably attachable to the chuck 5. However, the contact between the side walls 45 and the protrusions 15, and particularly the contact between the edges 50 and the protrusions 15, leads to wear and damage of the side walls 35, at least on the edges 50, and of the protrusions 15. Such wear, in turn, leads to less accurate positioning of the electrode relative to the chuck 5 in the x- and y-directions, and to less secure attachment of the electrode coupling 20 to the chuck 5.
The above discussion refers to the importance of positioning the electrode coupling 20 accurately relative to the chuck 5 in both the x- and y-directions. Note, however, that it is also important to position an electrode coupling 20 accurately relative to the chuck 5 in the z-direction. It is known to design the electrode coupling 20 such that a portion of the electrode coupling 20 contacts the surface 10 of the chuck 5, or a post 17 (see FIG. 4) on the surface 10, when the electrode coupling 20 is the required distance from the chuck S. In such designs, the protrusions 15 do not contact the floors 47 of the channels 40.
The above discussion refers to the attachment of electrodes to a chuck mounted on an electric discharge machine. It is similarly necessary to attach other tools to chucks mounted on other types of machines.